G. De Marzi

Design of an Industrially Feasible Twisted-Stack HTS Cable-in-Conduit Conductor for Fusion Application

Taking advantage of the large experience of the ENEA Superconducting Laboratory in the manufacture and characterization of large scale superconducting systems, a project was launched, aimed at using High Temperature Superconductor (HTS) 2G wires for the manufacture of a cable-in-conduit conductor (CICC). In particular, the main aim was the definition of a conductor design fully compatible with existing cabling technologies, to be promptly transferred to an industrial scale production. The considered layout is based on 150 HTS tapes, arranged as five layered structures of twisted tapes wound on a helically slotted core with external round jacket. All manufacturing steps (slotted core production, tape stacking and insertion into the ducts, external wrapping and jacketing) are fulfilled by using equipments and technologies available at TRATOS Cavi S.p.A. These CICCs are intended for operation using forced flow of Helium. A 2D local thermal model has been developed for the optimization of the cooling configuration. This conductor is designed to target 20 kA at 4.2 K and 15 T (or, alternatively, higher temperature, in self-field and LN2 cooling) corresponding to a Je of about 55 A/mm2. The production of a short dummy sample revealed that the exploited industrial production process is very promising for the development of HTS CICC.

The JT-60SA Toroidal Field Conductor Reference Sample: Manufacturing and Test Results

In the framework of the JT-60SA design activities, EU home team has defined a reference layout for the Toroidal Field conductor: it is a slightly rectangular Cable-In-Conduit NbTi conductor, operating at 25.7 kA with a peak field of 5.65 T. ENEA has assigned LUVATA Fornaci di Barga the task to produce the strands and to perform cabling, whereas jacketing and compaction have been carried out in its own labs. The sample, successfully tested at the CRPP SULTAN facility, has been assembled in such a way as to avoid the bottom joint between the two legs, thus using a single conductor length (about 7 m). An ad-hoc developed solution to restrain the U-bent conductor section (where jacket is not present), consisting in a stainless steel He-leak tight box with an inner structure designed in order to completely block the cable, has been also developed and manufactured by ENEA, where the sample has been also assembled. Instrumentation installation and final assembly of the sample have been performed by the SULTAN team. The main aspects of the sample manufacturing and characterization are here presented and discussed.

Cable-in-conduit conductors: lessons from the recent past for future developments with low and high temperature superconductors

We review progress in the design of high field superconducting cable-in-conduit conductors (CICCs) for fusion applications, with special attention to the results of recent key experiments, leading to the state-of-the-art CICC technology: the ITER Toroidal Field and Central Solenoid programs, the EFDA Dipole conductor development program, the NHFML Hybrid Magnet project, the EU-TF Alt conductor demonstration, and the CRPP React & Wind flat cable test. For these projects, the main CICC design driver was the mitigation of Nb3Sn conductor performance degradation with electro-magnetic loading cycles. This was achieved by proper choice of cable layout and of conductor geometry, depending on the specific operating conditions and project requirements. In all cases, the necessity to limit cable movements inside the conductor jacket was identified to be of crucial importance. The main aspects of CICC manufacture are also discussed here, at least for what is the experience gained by the authors in both CICC jacketing and cabling processes. Finally, the state of the art of high-temperature superconducting (HTS) cables is discussed: at present, this technology is still in its infancy, but it is highly likely that major technological improvements could eventually lead to a widespread use of HTS CICCs.

Electrical Characterization of ENEA High Temperature Superconducting Cable

ENEA is currently involved in the design and manufacture of a fully high temperature superconductor (HTS) cable in the cable-in-conduit conductor (CICC) configuration exploiting commercial second generation ReBaCuO (Re: Rare Earth and Y) coated-conductors. The final cable will be composed of five slots obtained in a helically twisted aluminum central core and filled with 2G tape stacks. This conductor is designed to operate above 10 kA in 12 T background field at 4.2 K or at about 10 kA in self-field at 77 K. A first sample of about 1-m length with one fully superconductive slot has been manufactured using 15 tapes provided by Superpower, Inc. and 12 tapes from the SuNAM Company grouped in two sub-stacks divided by a Kapton foil. Each tape of the stack has been characterized individually by measuring critical current values Ic at 77 K (liquid N2 bath) in self-field and n-index. Results revealed that the tapes showed no degradation of critical current values when compared with suppliers specifications confirming that the proposed manufacturing process is fully compatible with commercial coated-conductors. Inter-tape resistance(Rinter) has also been measured and the observed dependence of Rinter on the tape position in the stack has been put in correlation with transverse stress distribution calculated by finite element models. A second sample with a full superconducting slot has been manufactured using 18 SuNAM tapes. Preliminary results on the stack transport measurements performed at 77 K in self-field will be presented and discussed. All the samples were manufactured by using already existing industrial equipments at Tratos Cavi SpA.

Test Results of a NbTi Wire for the ITER Poloidal Field Magnets: A Validation of the 2-Pinning Components Model

A two-components model has been recently developed, for describing the normalized bulk pinning force curves and the critical current density of NbTi strands over a wider B-T range with respect to conventional single-component models. The model was previously successfully applied to data collected on several NbTi commercial strands, with different size, Cu:nonCu ratio, filament diameter and layout, thus confirming the presence of two different pinning mechanisms in conventionally processed NbTi wires. For a further validation, we have extensively tested a strand recently produced by the Chinese Company Western Superconducting Technologies for the ITER Poloidal Field (PF) magnets PF2 to PF5, and applied the model to these data. In order to take into account the observed non-scaling with temperature of the reduced pinning force curves, the model has been updated, including the observed difference in the temperature dependences of the two components contributing to the overall bulk pinning force. The importance of testing wires over very wide temperature ranges is evidenced, and the good agreement between experimental and fit results validate the proposed formulation, which can be regarded as a reliable tool for the description of NbTi performances, to be used in the design of superconducting magnets. From the phenomenological point of view, it is shown that at low temperatures, the two pinning mechanisms contribute to the bulk pinning force, resulting in a pinning force peaking at a reduced field B/Birr ≅ 0.5. As the temperature increases, the pinning force peak moves to lower fields, indicating that the low field component pinning mechanism becomes dominant.

Strain sensitivity and superconducting properties of Nb3Sn from first principles calculations

Using calculations from first principles based on density-functional theory we have studied the strain sensitivity of the A15 superconductor Nb3Sn. The Nb3Sn lattice cell was deformed in the same way as observed experimentally on multifilamentary, technological wires subject to loads applied along their axes. The phonon dispersion curves and electronic band structures along different high-symmetry directions in the Brillouin zone were calculated, at different levels of applied strain, ε, on both the compressive and the tensile side. Starting from the calculated averaged phonon frequencies and electron-phonon coupling, the superconducting characteristic critical temperature of the material, T(c), has been calculated by means of the Allen-Dynes modification of the McMillan formula. As a result, the characteristic bell-shaped T(c) versus ε curve, with a maximum at zero intrinsic strain, and with a slight asymmetry between the tensile and compressive sides, has been obtained. These first-principle calculations thus show that the strain sensitivity of Nb3Sn has a microscopic and intrinsic origin, originating from shifts in the Nb3Sn critical surface. In addition, our computations show that variations of the superconducting properties of this compound are correlated to stress-induced changes in both the phononic and electronic properties. Finally, the strain function describing the strain sensitivity of Nb3Sn has been extracted from the computed T(c)(ε) curve, and compared to experimental data from multifilamentary, composite wires. Both curves show the expected bell-shaped behavior, but the strain sensitivity of the wire is enhanced with respect to the theoretical predictions for bulk, perfectly binary and stoichiometric Nb3Sn. An understanding of the origin of this difference might open potential pathways towards improvement of the strain tolerance in such systems.

Direct observation of Nb3Sn lattice deformation by high-energy x-ray diffraction in internal-tin wires subject to mechanical loads at 4.2 K

With the aim of clarifying the relationship between lattice deformations and superconducting properties of Nb3Sn technological wires we have carried out high-energy x-ray diffraction experiments at the European Synchrotron Radiation Facility (ESRF) in Grenoble on individual samples of multi-filamentary internal-tin-type Nb3Sn wires. In particular, a test probe developed at the University of Geneva allowed us to perform these experiments at 4.2 K, while applying an axial tensile load to the specimen. In this way, the lattice parameter values of all the constituents (Nb3Sn, Nb, Cu) were determined, in both the parallel and orthogonal directions with respect to the applied load axis, as a function of the applied strain. The experiments were performed on industrial wires, which were reinforced by a stainless steel outer tube, applied before the Nb3Sn reaction heat treatment, in order to evaluate the effect of an additional pre-compression strain. The relation between the microscopically determined crystalline lattice deformations and the measured applied strain is discussed as a basis for the analysis of the superconducting performances of Nb3Sn wires subject to mechanical loads.

Manufacturing of the ITER TF Full Size Prototype Conductor

The experience gained in the past for the ITER toroidal field model coil conductor and the results obtained so far have led to the definition of an upgraded full size prototype conductor, based on advanced Nb3Sn strand, and entirely manufactured in the European Union (EU). Samples for the characterization in the Sultan facility have been prepared by Luvata (Italy) following the conductor layout defined by ITER. ENEA was responsible for conductor fabrication. Since the conductor layout was new, a full size copper dummy conductor has been preventively produced for the setting of the cabling and jacketing tools. Then, a total of four full size superconducting cables have been prepared by using Nb3Sn advanced strands produced by Oxford Instruments (OST) and European advanced superconductors (EAS), by internal tin and bronze technology, respectively. The details of manufacturing procedures will be described in this paper.

Improvement of electromechanical properties of an ITER internal tin Nb3Sn wire

The critical current of an internal tin Nb3Sn wire developed by Oxford Instruments, Superconducting Technology for International Thermonuclear Experimental Reactor (ITER) (OST type-I, billet No. 7567) has been studied under axial strain at fields between 12 and 19 T at 4.2 K. Simulating the situation in a cable in conduit, where thermally induced compressive strain is important, a single wire (strand) was jacketed with AISI 316L stainless steel. The reinforced wire shows an important increase in em, the applied strain where Ic reaches its maximum, from 0.25% to 0.57%. In addition the irreversibility limit, eirr, is improved from 0.50% applied strain to >1.10%. It could also be shown that the Ic at zero intrinsic strain is almost identical. This demonstrates that jacketing does not influence the physical parameters of the original wire. Experimental data of the bare wire has been well fitted by different strain functions. However, it was not possible to model the data of the jacketed wire. There are indica...

Influence of cable layout on the performance of ITER-type Nb3Sn conductors

In 2005-2006, the European Union tested in the SULTAN facility (CRPP Villigen, Switzerland) four ITER-type conductors assembled in two samples. Each conductor made use of an advanced Nb3Sn strand specially developed by European companies for the ITER Toroidal Field (TF) coil conductor. The cable layout of these conductors was the one of the ITER TF Model Coil conductor. In 2007, the European Union tested in SULTAN four other conductors within two other samples as prototypes of the updated ITER TF conductor. These conductors differed mainly from the former by their cable layout whereas identical or similar Nb3Sn strands were used. The paper reports on the current sharing temperature measurements of the second samples under ITER relevant operating conditions. The effect of cycling (1000 cycles) in current under full magnetic field was also investigated. The analysis of the test results is performed with respect to the strand properties using the usual smeared models of the cable. These test results were found to be significantly better than those obtained on the first conductors. The paper discusses the possible causes as well as the consequences of such a performance scattering.